The largest of the planets in our Solar System, the name
Jupiter was an accident since the ancient astronomers did not know
Jupiter's real size. Its radius is 11.3 Earth radii, its mass is 317
Earth masses. It is composed mostly of hydrogen (90%) and helium (9%) and
traces of everything else. Jupiter's mean density is 1.3 gm/cc, close to
that of water.

As we discussed before, the mean density of the Jovian worlds is near
the value for water, 1 gm/cc, versus the terrestrial worlds which
have average densities near the value of rocks, 3 to 5 gm/cc. This
was due to the fact that temperatures in the outer Solar System are
low because of the large distance from the warm Sun. So volatile
compounds, such as ices like H2O, CO2,
NH3, CH4, which tended to evaporate in the
inner Solar System (although not all of them since there was plenty
leftover to form secondary atmospheres) are abundent in the outer
Solar System and make-up most of the comets, moons and rings around
the Jovian worlds.

Note that H2O (water), CO2 (carbon dioxide),
NH3 (ammonia) and CH4 (methane) are the
simplest molecules you can make with hydrogen (H), carbon (C), oxygen
(O) and nitrogen (N) = often called the HCNO compounds. Jupiter is
also rich in NH4SH = ammonium hydrosulfide.

Jupiter's Formation:

The formation of Jupiter (and the other Jovian worlds) starts with the
accretion (build-up) of ice-covered dust in the outer, cold solar
nebula

Note that, due to gravity, the heavier elements sink to the core of
proto-Jupiter, thus, we expect the core region to be rocky. While the
lighter elements (H and He) remain in the atmosphere.

Jupiter's Atmospheric Features:

The exterior of Jupiter is noted by its brightly colored latitudinal
zones, dark belts and thin bands dotted with numerous storms and eddies.
Due to differential rotation, the equatorial zones and belts rotate faster
than the higher latitudes and poles as seen in this
Jupiter movie.

The zones and belts are zonal jet streams moving with velocities up to
400 miles/hr. Wind direction alternates between adjacent zones and
belts. The light colored zones are regions of upward moving convective
currents. The darker belts are made of downward sinking material. The
two are therefore always found next to each other. The boundaries of the
zones and belts (called bands) display complex turbulence and vortex
phenomenon.

Gas planets do not have solid surfaces, but rather build-up in pressure
and density as one goes deeper towards the core. Different colors
represent different depths into Jupiter's atmosphere. The colors (reds,
browns, yellows, oranges) are due to subtle chemical reactions involving
sulfur. Whites and blues are due to CO2 and H2O ices.

The detailed structure in
Jupiter's atmosphere is dominated by physics known as fluid mechanics. Note
that the atmosphere of Jupiter is so dense and cold that it behaves as a
fluid rather than a gas. At the point were we see atmospheric features the
pressure is 5 to 10 times that of the Earth's atmospheric
pressure at sea level.

The simplest theories in fluid mechanics
predict two types of patterns. One pattern occurs when a fluid slips
by a second fluid of a different density. Such an event is known as
a viscous flow and
produces wave-like features at the boundary of the two fluids. A
second pattern is produced by a stream of fluid in a constant medium,
called turbulent
flow. The stream breaks up into individual elements, called eddies. These eddies can
develop into cyclones.

Cyclones develop due to the Coriolis effect where
the lower latitudes travel faster than the higher latitudes producing
a net spin on a pressure zone. The cyclones on Jupiter are regions
of local high or low pressure spun in such a fashion. Note that the
direction of the spin differs in the two hemispheres where
clockwise spin is in the North and counter-clockwise spin is in the
South.

Brown ovals are low pressure cyclones/storms in the North. White ovals
are high pressure cyclones/storms in the South. Both can last on the
order of tens of years. The Great Red Spot is a large high pressure
storm that has lasted over 600 years.

On the Earth, the energy to power our storm systems comes from sunlight.
Jupiter is too far from the Sun and receives very little energy. The
energy needed to power all the turbulence in Jupiter's atmosphere comes
from heat released from the planet's core.

Jupiter's interior:

Jupiter is highly oblate (flattened). Plus, Jupiter has a very high
rotation rate (once every 9.8 hours). These two facts combine to tell us
that Jupiter has a very small solid core.

Jupiter's interior consists mostly of hydrogen and helium. These
elements are gaseous at the top of Jupiter's atmosphere down to several
thousand kilometers. At this point, the pressures and temperatures
compress these gases into a liquid state.

The liquid hydrogen, in molecular form at these levels
(H2), continues to be compressed further reaching a
metallic state. This occurs in a transition zone located 20,000 km
below the atmosphere. Notice that at no time is there any real
``surface'' as one drops into Jupiter's interior.

At the very center of Jupiter is a small (15 Earth masses) rocky
core, leftover from the icy dust particles that originally collected
in the early solar nebula.

A planet absorbs energy from the Sun in the form of light and converts the
energy into heat. The heat is then reradiated back into space (mostly
from the nightside of the planet). Based on how much energy Jupiter
absorbs from the Sun, then its mean temperature should be 105 K (about
-280 F). However, IR and radio measurements of Jupiter show that it has a
mean temperature of 125 K, or 20 degrees too warm. In other words,
Jupiter radiates about twice as much energy as it receives. Conservation
of energy requires that this heat come from someplace and the only
reservoir is the core of Jupiter. Thus, this extra heat is leftover energy
from the time of Jupiter's formation.

Many textbooks refer to Jupiter as a ``failed star''. This is due to the
fact that if Jupiter were slightly more massive the temperatures in its
core would have reached the ignition point for thermonuclear fusion.
This is the process where stars turn hydrogen into helium and release
energy (i.e. the star shines). If Jupiter were 100 times more massive,
our Solar System would have had two stars.

Jupiter's radiation output:

IR and radio measurements revealed two components to Jupiter's
radiation output; a thermal and non-thermal component.

A thermal component is associated with the leftover heat of formation
(see above). The non-thermal component is associated with radiation
that does not follow a Planck
curve but follows what is know as a power-law spectrum. A
spectrum that associated with synchrotron
radiation.

Jupiter's magnetic field:

The magnetic field
of Jupiter is 19,000 times stronger than the Earth's magnetic field.
Even with a large rocky core and high rotation rate, the magnetic
field is too strong. The origin of Jupiter (and other Jovian
planets) strong magnetic field is the metallic hydrogen shell that
surrounds Jupiter's rocky core. Metal is an excellent conductor of
electric current and supplies the energy for the generation of an
intense and large magnetic field.

A strong magnetic field can capture charged particles from the solar
wind (i.e. high speed protons and electrons) and particles ejected from
the inner moon, Io. These particles are trapped in the inner magnetic
belts and are reflected back and forth between the north and south
magnetic poles.

A visible result of this interaction is
aurora or northern lights on Jupiter.

The interaction of Jupiter's strong magnetic field and nearby space
produces a region known as Jupiter's magnetosphere. The magnetosphere
has several features:

North and South magnetic poles

current sheet along the magnetic equator

plasma torus associated with Io's orbit

magnetopause - the boundary where the magnetosphere encounters the
solar wind

The rapid rotation of Jupiter spews charged particles into a current
sheet around the magnetic equator of Jupiter. Inside this current
sheet orbits the moon Io. The current sheet sweeps up ejected ions from
Io's geysers to make a plasma torus. The region around this plasma
torus and the inner moon system is intensely radioactive with levels
around 1000 times the radioactive levels of the Earth's surface. This
region of space is inhabitable by man or machine without heavy
shielding.

The magnetosphere of Jupiter encounters the solar wind at about a
million kilometers from the planet. The bow shock from this boundary
reaches beyond the orbit of Saturn.